Methods and apparatus for an improved discrete LC filter

ABSTRACT

A discrete inductive-capacitive (LC) filter selects between at least two inductor banks to tune the LC filter. The filter receives an input signal that includes one or more bands of frequencies. A control signal selects a band of frequencies for processing. A first inductor bank is selected to filter a first band of frequencies, and a second inductor bank is selected to filter a second band of frequencies. A switch circuit couples the input signal to either the first inductor bank or the second inductor bank. The switch circuit selects the first inductor bank if the first band of frequencies is selected, and selects the second inductor bank if the second band of frequencies is selected. The switch circuit electrically isolates the switching of the input signal to the first and the second inductor banks, so as to enhance the Q factor of the LC filter. Circuit and techniques are disclosed to reduce parasitic capacitance in a capacitive bank that employs MOS transistors. Furthermore, circuits and techniques are disclosed to tune a coupling factor on an inductive bank based on the frequency of the input signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed toward the field of discrete filters,and more particularly toward capacitor and/or inductor bank filters.

2. Art Background

Inductor and capacitor banks may be configured to implement manydifferent types of discrete filters. FIG. 1 a illustrates one embodimentfor an inductive (L) bank. For this embodiment, the inductive bankincludes five inductors (110, 108, 106, 104 and 102). Although inductivebank 100 includes five inductors, any number of inductors may be usedwithout deviating from the spirit or scope of the invention. The numberand values for the inductors is a function of the desired frequencyresponse characteristics of the filter. The inductors, which forminductive bank 100, are configured in parallel. Each inductor is addedto the L bank through a corresponding switch as shown in FIG. 1 a.Typically, the switches are implemented using metal oxide semiconductor(“MOS”) transistors.

FIG. 1 b illustrates one embodiment for a capacitive bank. For thisexample, capacitive bank 120 contains five capacitors (130, 128, 126,124 and 122). A different number of capacitors and different capacitivevalues may be selected to implement different frequency responses. Also,as shown in FIG. 1 b, capacitors 128, 126, 124 and 122 are selected forthe C bank through a respective switch. Typically, these switches areimplemented with MOS transistors.

Each MOS switching transistor introduces a resistive component into thefilter response. Thus, each capacitor selected in the C bank increasesthe series resistance. The increase in series resistance, or decrease inparallel resistance, decreases the Q factor, which, in turn, degradesperformance of the filter bank.

Accordingly, it is desirable to improve the characteristics andperformance of an LC filter by reducing parasitic capacitance andincreasing the Q factor.

SUMMARY OF THE INVENTION

A discrete inductive-capacitive (LC) filter selects between at least twoinductor banks to tune the LC filter for one or more bands offrequencies. The filter receives an input signal for processing. Theinput signal includes one or more bands of frequencies. A control signalselects a band of frequencies for processing. A first inductor bank,which comprises at least one inductor, is selected to filter a firstband of frequencies, and a second inductor bank, which also comprises atleast one inductor, is selected to filter a second band of frequencies.A switch circuit couples the input signal to either the first inductorbank or the second inductor bank. The switch circuit selects the firstinductor bank if the first band of frequencies is selected, and selectsthe second inductor bank if the second band of frequencies is selected.The switch circuit electrically isolates the switching of the inputsignal to the first and the second inductor banks, so as to enhance theQ factor of the LC filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates one embodiment for an inductive (L) bank.

FIG. 1 b illustrates one embodiment for a capacitive bank.

FIG. 2 is a block diagram illustrating one embodiment for implementingLC filters.

FIG. 3 illustrates one embodiment for an input stage of an amplifier.

FIG. 4 illustrates one embodiment for an output stage an amplifier thatselects between inductor banks.

FIG. 5A illustrates one embodiment for electrically coupling capacitors.

FIG. 5B illustrates one embodiment for reducing parasitic capacitancefor a capacitor bank.

FIG. 6A illustrates an example inductor bank.

FIG. 6B illustrates an inductor bank configured in accordance with oneembodiment of the present invention.

FIG. 7 illustrates a frequency response for an example LC filter.

FIG. 8 illustrates an example LC filter.

DETAILED DESCRIPTION

FIG. 2 is a block diagram illustrating one embodiment for implementingLC filters. Circuit 200 includes two signal paths: a signal path for afirst band (e.g., Band I), and a signal path for a second band (e.g.,Band II, III). In one application, circuit 200 comprises LC filter banksfor a television tuner. For the television tuner embodiment, the signalpath for “Band I” filters input signals within the frequency range of 50to 285 MHz, and the signal path for “Band II, II” filters input signalswithin the frequency range of 285 to 880 MHz.

The signal path for the first band consists of an inductive bank (i.e.,transformer) comprising inductors 206 and 208. The inductive bank(inductors 206 and 208) receives the input signal from radio frequency(“RF”) input 202. Circuit 200 further includes, in the first signalpath, capacitor bank 220. Capacitor bank 220 comprises a plurality ofcapacitors selectively coupled to the first signal path. The selectivecoupling of capacitors in capacitor bank 220 along with inductors 206and 208 comprise a first tunable LC filter. The output of capacitor bank220 is input to an amplifier (e.g., automatic gain controlled) 226.

The RF input 202 is also coupled to the second signal path, throughcapacitor 204, for the second bank of input frequencies (e.g., Bands II,III). The second signal path for the second band consists of an inductorbank, consisting of inductors 210 and 212, and a capacitor bank 222.Capacitor bank 222 comprises a plurality of capacitors selectivelycoupled to filter the signal in the second signal path. The capacitorsof capacitor bank 220 and inductors 210 and 212 comprise a first tunableLC filter for the second signal path. The output of capacitor bank 220is input to AGC amplifier 226.

The AGC amplifier 226 selectively couples either the first signal pathor the second signal path to the output of amplifier 226. For example,in the television tuner embodiment, AGC amplifier 226 selects the firstsignal path if the television tuner is set to tune a channel in Band I.Alternatively, AGC amplifier 226 selects the second signal path if thetelevision tuner is set to tune a channel in Bands II, III. Oneembodiment for AGC amplifier 226 is described more fully below inconjunction with a discussion of FIGS. 3 and 4.

For the first signal path, the output of AGC amplifier 226 is coupled tocapacitor bank 228. The output of capacitor bank 228 is input to aninductor bank. In turn, the output of the inductor bank is input tocapacitor bank 240. For this embodiment, the inductor bank comprisesinductors 234 and 236, and tunable capacitors 230 and 232. Thecapacitors of capacitor banks 228 and 240, similar to capacitor bank220, are selected to tune the LC filter. The output of the first signalpath is input to buffer 254.

For the second signal path, the output of AGC amplifier 226 is coupledto capacitor bank 242. Similar to the first signal path, the output ofcapacitor bank 242 is input to an inductor bank (i.e., inductors 248,250 and tunable capacitors 230 and 246). The output of the inductor bankis input to capacitor bank 252. The capacitors of capacitor banks 228and 240 are selected to tune or program the LC filter. The output of thesecond signal path is also input to amplifier 254.

The inductors, for the embodiment of FIG. 2, do not include switches(e.g., MOS transistors), coupled in series with the inductors, forselectively adding inductance to an LC filter. Instead, the input signal(i.e., the signal for processing) is switched in an amplifier betweensignal paths (e.g., the first and second signal paths for the embodimentof FIG. 2). The elimination of the series resistance in the inductorbank results in a better Q factor for the LC filter. The amplifier,which switches between the first and second signal paths, isolates theresistance from the MOS transistors. Although the embodiment of FIG. 2isolates the switches in the input and output transistor stages, anyconfiguration to isolate the series resistance of an electronic switchfrom inductors may be used without deviating from the spirit or scope ofthe invention.

The LC filter architecture of FIG. 2 minimizes the number of coils. Thefirst and second signal paths are suitable for covering a wide range ofinput frequencies. For example, in the embodiment of FIG. 2, only twocoils are used for both the first and second signal paths. Thisarchitecture has application for use in processing UHF/VHF bands oftelevision signals.

FIGS. 3 and 4 illustrate one embodiment for input and output stages,respectively, of an amplifier. The amplifier switches between signalpaths to effectively select inductor banks for an input signal. Ingeneral, the amplifier receives, as input, signals from the first andsecond signal paths as well as control signals (band and band^(/)). Forthe first signal path (e.g., signal path for Band I), transistors 328and 332, when selected, drive the output for this transistor stage. Ifthe second signal path is selected (e.g., signal path for Band II, III),transistors 318 and 320 drive the output for this transistor stage. Thedifferential input signal path includes, for the first signal path,capacitors 326 and 330, and capacitors 318. The second signal pathreceives a differential signal for input to capacitors 314 and 316.

The control signal, Band, controls the switching of switch 304, for thefirst signal path, and controls the switching of switches 338 and 340 inthe second signal path. The control signal, Band^(/), has a valueopposite from the control signal Band. The control signal Band^(/)controls, for the first signal path, the switching of switch 302, andcontrols the switching of switches 322 and 324 in the second signalpath. In one embodiment, the switches (302, 304, 322, 324, 338 and 340)comprise metal oxide semiconductor (MOS) transistors. In operation, toselect the first signal path (e.g., Band I), Band is set to a low logiclevel and Band^(/) is set to a high logic level. Under these controlsignals, switch 302 is turned on, and switches 338 and 340 are turnedoff. The activation of switch 302 biases the output transistors (328 and332) to conduct. As a result, the input signal is conducted throughtransistors 328 and 332. Also, a low logic level signal on Band turnsoff switch 304, and a high logic level on Band^(/) turns on switches 322and 324. When closed, switches 322 and 324 pull the base of transistors318 and 320 to ground, and the input signal to the second signal path isnot passed to the output of the input transistor stage (300).

Conversely, when Band is set to a high logic level and Band^(/) is setto a low logic level, switch 302 is opened and switches 338 and 340 areclosed. Under these control signals, the voltage level at the bases oftransistors 332 and 328′ are pulled to ground, thus turning offtransistors 328 and 332. As a result, the input signal from the firstsignal path is not passed to the output of the input transistor stage(300). Also, a high logic level on Band and a low logic level onBand^(/), closes switch 304 and opens switches 322 and 324. Theactivation of switch 304 biases the output transistors, 318 and 320,through pull-up of resistors 308 and 306, to conduct. As a result, theinput signal to the second signal path is passed to the output of theinput transistor stage (300). Each of the output lines of thistransistor stage includes a current buffer, illustrated on FIG. 3 ascurrent sources 334 and 336.

FIG. 4 illustrates one embodiment for an output stage an amplifier thatselects between inductor banks. The amplifier isolates the switching oftransistors and consequently isolates transistor series resistance tothe inductor banks. In one embodiment, the output stage of the amplifiercomprises a transconductance (g_(m)) amplifier. For this embodiment, thetransconductance amplifier, which converts a voltage to a current,comprises transistors 410, 412, 414, 416, 418 and 420, as well asvariable resistors 422 and 426 and current source 428. To select thefirst signal path (e.g., Band I), the Band control signal is set to alow logic level, and the Band^(/) control signal is set to a high logiclevel. A high logic level Band^(/) control signal turns on transistor404 to place a high logic level at the base of transistors 410 and 412.Consequently, transistors 410 and 420 drive the output for Band I or thefirst signal path. Also, a high logic level on Band^(/) closes switch406, pulling the base of transistors 412 and 418 to ground. As a result,transistors 412 and 418 are turned off, and the input does not pass tothe second signal path (Band II, III).

Alternatively, to select the second signal path (Bands II, III), theBand control signal is set to a high logic level, and the Band^(/)control signal is set to a low logic level. A high logic level Band^(/)control signal turns on transistor 404 to place a high logic level atthe base of transistors 410 and 412. As a result, transistors 412 and418 drive the output for the second signal path (Band II, III). Also, alow logic level on Band^(/) closes switch 408, pulling the base oftransistors 410 and 420 to ground and turning the transistors off (i.e.,the input does not pass to the first signal path (Band I) to theoutput).

FIG. 5A illustrates one embodiment for electrically coupling capacitors.In general, one or more capacitors selectively couple a plurality ofcapacitors to form a configurable capacitor bank. For the examplecircuit 500 of FIG. 5A, capacitors 502 and 506 are coupled through aswitch 504: In one embodiment, switch 504 comprises a metal oxidesemiconductor (MOS) transistor. A control signal (CT) enables switch 504so as to electrically connect capacitors 502 and 506.

FIG. 5B illustrates one embodiment for reducing parasitic capacitancefor a capacitor bank. An example capacitor bank includes capacitors 508and 512 selectively coupled by MOS transistor (e.g., NMOS) 510. Tominimize the parasitic capacitance, the circuit of FIG. 5B includestransistors 540 and 550. A control signal (CT) is used to selectcapacitors (508 and 512) to configure a capacitor bank. The controlsignal (CT) is input to the gates of transistors 540 and 550 as well asthe gate of transistor 510 through resistor 520. Transistor 540, whenturned on, couples node 560, through resistor 514, to ground. As shownin FIG. 5B, node 560 is located at a point connecting transistor 510 andcapacitor 508. Also, transistor 550 connects, when activated by CT, node570 to ground through resistor 530. Although the circuit shown in FIG.5B may be used to selectively couple two capacitors, the circuit may beduplicated to selectively couple any number of capacitors withoutdeviating from the spirit and scope of the invention.

In operation, to connect capacitors 508 and 512, CT is set to a highlogic level. A high logic level on CT turns on transistor 510 to couplecapacitors 508 and 512. Also, a high logic level on CT turns transistor540 on and transistor 550 off. Therefore, transistor 540 lowers thevoltage at nodes 560 and 570 by pulling the voltage toward groundthrough resistors 514 and 530, respectively. In this state (i.e.,transistor 510 is turned on), the voltage at nodes 560 and 570 properlybias the transistor. To de-couple capacitors 508 and 512, the CT signalis set to a low logic level. As a result, transistors 540 and 510 areturned off and transistor 550 is turned on. The activated transistor 550increases the voltage at nodes 560 and 570 based on resistors 514 and530 and the bias voltage at transistor 550. Also, a low logic level onCT grounds the voltage at the gate of transistor 510. The increased gateto source voltage of transistor 510, a result of the voltage at thesource (i.e., node 570) and the voltage at the gate, reduces thegate-source junction capacitance of transistor 510. Similarly, theincreased gate to drain voltage of transistor 510, a result of thevoltage at the drain (i.e., node 560) and the voltage at the gate,reduces the drain-source junction capacitance of transistor 510. Thus,the circuit minimizes parasitic capacitance generated from the drain togate and gate to source junctions on the MOS transistors.

In one embodiment, the filter-characteristics of an inductive bank areimproved. Specifically, the bandpass characteristic of an inductor bankis improved by tuning capacitance across inductors based on desiredcharacteristics of the filter (e.g., capacitance is selected to adjust afilter based on tuning frequency of a receiver). FIG. 6A illustrates anexample inductor bank. As shown in circuit 600 of FIG. 6A, inductors 610and 620 are configured in parallel. The response of the inductor bank600 is based on, in part, a mutual coupling factor between inductors 610and 620. The mutual coupling factor is illustrated in FIG. 6A by theline with arrows and the symbol, M.

FIG. 6B illustrates an inductor bank configured in accordance with oneembodiment of the present invention. For this embodiment, the inductorbank comprises inductors 640 and 650. In addition, the inductor bank 630comprises variable capacitor 660. The variable capacitor 660 is tunable,such that the capacitance introduced may be varied. The variablecapacitor 660 may comprise any type of device capable of generatingvariable capacitance.

In general, the coupling factor for the inductor bank (e.g.,transformer) is controlled by introducing capacitance across theinductors (e.g., variable capacitor 660 in circuit 630 of FIG. 6B). Thiscapacitance is selected based on a tuning frequency of the LC filter. Incontrast, the coupling inductor bank 600 of FIG. 6A is based on theinductors' mutual coupling factor. By selecting a capacitance to tunethe coupling factor, a constant bandwidth across variable LC filtercharacteristics (i.e., center frequencies) may be achieved.

FIG. 7 illustrates a frequency response for an example LC filter. Forpurposes of nomenclature, the bandpass response is characterized by acenter frequencies f_(c), f_(c1) and f_(c2). FIG. 8 illustrates anexample LC filter. The following terms are used to define variousrelationships in the LC filter:

-   -   BW—Bandwidth    -   k_(c)—Capacitive coupling    -   k₁—Inductive coupling    -   M—Mutual inductance

The center frequency, fc, may be defined in accordance with theexpression:fc{square root}{square root over (fc1*fc2)}.The relationship between the frequencies fc1 and fc2 is based on thebandwidth of the response such that:fc1=BW+fc2.The capacitive coupling factor may be expressed as:${{kc} = \frac{Ck}{C}},$and the inductive coupling factor may be expressed as:${kl} = \frac{M}{L}$wherein, “C” and “L” are the capacitances and inductances shown in FIG.7. The total coupling factor may be expressed as a sum of the capacitiveand inductive coupling:k=kc+kl.The total coupling factor may be expressed as a function of the fc1 andfc2 frequencies:$k = \frac{\left( \frac{fc1}{fc2} \right)^{2} - 1}{\left( \frac{fc1}{fc2} \right)^{2} + 1}$Accordingly, the center frequency range (i.e., fc1 to fc2) of thebandpass response of the LC filter is tunable based, in part, on thevalue of the capacitive coupling factor.

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that various modificationsand alterations might be made by those skilled in the art withoutdeparting from the spirit and scope of the invention.

1. A tunable discrete LC filter comprising: an input for receiving aninput signal for processing, said input signal comprising a plurality offrequencies; control input for receiving information to select at leastone band of frequencies for processing; first inductor bank forfiltering a first band of frequencies; second inductor bank forfiltering a second band of frequencies and switch circuit, coupling saidinput signal to said first inductor bank and said second inductor bank,so as to electrically isolate said switching of said input signal tosaid first inductor bank and said second inductor bank, respectively,said switch circuit for selecting said first inductor bank if said firstband of frequencies is selected, and for selecting said second inductorbank if said second band of frequencies is selected.
 2. A method fortuning a discrete LC filter, said method comprising the steps of:receiving an input signal for processing, said input signal comprising aplurality of frequencies; receiving information to select at least oneband of frequencies for processing; switching said input signal to afirst signal path if a first band of frequencies was selected; couplinga first inductor bank to said first signal path; electrically isolatingsaid switching of said input signal from said first inductor bank;filtering said first band of frequencies in said first inductor bank;switching said input signal to a second signal path if a second band offrequencies was selected; coupling a second inductor bank to said secondsignal path; electrically isolating said switching of said input signalfrom said second inductor bank and filtering said second band offrequencies in said second inductor bank.